Well, it's an old car, and stuff like this is to be expected.
Possible reasons could include:
- Clogged fuel filter
- Bad fuel pump
- Defective lambda probe
- Bad spark plugs and/or spark plug wires...
- (I could go on for ages...)
I was digging trough BMWslo forum, and I found a great post about this...
In some cases, the relay that is providing power to the injectors can overheat, and the injectors could lose power for a moment. (White relay on the picture) - That relay is the same on all BMW's from E28 onward, so you can easily swap it.
So, this is a new info for me, and IMO, a very useful one...
I'll change the relay today, and see if this is the case... If not, I already bought a new fuel filter, and I have 2 spare lambda probes at home... So, I'll keep you updated.